Antiship torpedoes constitute a danger to the ships of nations depending on imports or relying heavily on foreign commerce. They are the major threat to the noncombatant ships, particularly, where they are unescorted and vast amounts of war materials must be rapidly delivered to stem a confrontation.
Therefore, it becomes imperative that systems be developed for detecting and neutralizing torpedoes.
Straight running torpedoes, as opposed to being homing or guided, typically are dodged by sharply maneuvering a ship out of their paths. Usually, early detection or an awareness that a torpedo has been fired is needed so that random evasive action can be taken. This Russian roulette approach sometimes works, sometimes doesn't.
While studies indicate that a certain percentage of ships escape damage by the maneuvering exercise outlined above, blind turns do not deceive homing or guided torpedoes. Some acoustic homing torpedoes can be jammed by some acoustic noise jammers; yet, these jammers don't affect straight running torpedoes nor the signature homing torpedoes. Particularly with respect to the latter, synthesizing or masking a ship's signature is a complicated, expensive procedure and the results have fallen short of expectations.
Ship-length barriers have been proposed. These envelope a ship in a protective mesh or other suitable barrier like material and are either continuously dragged along in the water or are thrown overboard when there is a threat. Even in the case when the barriers are to be deployed after an antiship torpedo has been detected, the barriers tend to be ponderous and compromised ships' performance by slowing them down and increasing fuel consumption. Furthermore, after a torpedo has been detected, ballistic missiles, high-velocity, flat trajectory missiles, barriers of hovering charges with proximity fuses or hedgehog depth bombs are marginal since contemporary acoustic ranging techniques fail to provide a precise location of an incoming torpedo. Detection and tracking of the oncoming torpedoes by the latest towed tandem arrays leaves much to be desired for reliable interception.
Because of the limitations of the acoustic detectors, optical antiship torpedo detection systems are being investigated. A light detection and ranging (LIDAR) system has been proposed for detecting shallow submerged objects so that a hydrofoil boat, for example, could take evasive action. A paper entitled "Grazing Angle LIDAR for Detection of Shallow Submerged Objects" was presented at the Dec. 11, 1978, meeting of the Optical Society-IEEE Laser Conference in Florida by Richard D. Anderson, Robert F. Howarth, and Gregory C. Mooradian. The LIDAR system disclosed at the conference, while a meritorious advance in the state-of-the-art, did not meet all expectations even though detection of some near surface objects was obtained by reflections of energy in the blue-green spectrum. Precise localization needs to be improved and the depth of detection needs to be increased to detect torpedoes running at depths down to sixty feet. The extraction of the signal from the clutter signal created by surface-return and volume-backscatter-seawater-return also posed problems which limited its effective depth.
Airborne methods of optically detecting submerged objects have been attempted. One is disclosed by Elliot H. Kahn in his U.S. Pat. No. 3,604,803 and is entitled "Optical Detection Method for Submerged Objects". He proposes an aircraft flying above the water and vertically directing a blue-green laser beam and sensing the reflections to detect the submerged object. While this method may meet with a certain degree of success, it cannot be employed in the manner envisioned by the method of this invention which concerns itself with a ship effecting the detection of a torpedo at a relatively small grazing angle with respect to the water's surface and the subsequent neutralization of the torpedo. Backscattering in the present instance is of a magnitude far greater than that tolerated by the Kahn method.
Thus, there is a continuing need in the state-of-the-art for a method and system for assuring the detection and location of an antiship torpedo by the target ship which effectively allows the neutralization of the torpedo.